1. Field of the Invention
The present invention generally relates to photonic amplitude modulators. In particular, a photonic modulator having separate diodes for tuning and modulating functions.
2. Description of the Related Art
Photonic amplitude modulators based on interferometric or resonant waveguide structures modulate light by introducing a change in the effective index of refraction, which provides a shift in the optical phase of a lightwave passing through the modulator. This index change is often accomplished by implementing a phase shifter diode across the waveguide and operating the diode in either the forward-biased or reverse-biased condition. In the forward-biased condition, the diode injects a comparatively large amount of current at low voltage, inducing a large index shift for a given length of waveguide. This means that the phase shifter diode can be made using a relatively short length of waveguide. However, the inherently large charge density within the diode results in lower speed performance.
Alternatively, in the reverse-biased state, only small leakage currents flow through the diode, and the electric field primarily induces the change in index. As a result, the diode may operate much faster under this condition, but because the index shift per unit waveguide length is smaller than in the forward-biased region, either a longer waveguide or a much larger voltage is required to generate the needed phase shift. In practice, longer waveguides and much larger voltages are applied to reverse-biased diodes compared to their forward-biased counterparts.
Consequently, a trade-off between power, speed, and area exists when choosing a forward-biased or reverse-biased diode design. A forward-biased diode may be optimized for area by making the waveguide short and operating the diode at increased voltage and current values in order to generate a larger index shift per unit waveguide length. Alternately, a forward-biased diode may be optimized for power by making the waveguide somewhat longer and operating the diode at reduced voltage and current values generating a smaller index shift per unit waveguide length, which accumulates over a longer length of waveguide. Finally, a forward-biased diode may be designed to implement a compromise between power and area by choosing a waveguide length somewhere in between the power-optimized and area-optimized design points.
A Mach-Zehnder interferometer (MZI) is often used to modulate light waves to transmit data. The Mach-Zehnder interferometer uses interference to transform the aforementioned phase modulation into amplitude modulation. Normally both arms of the Mach-Zehnder interferometer are made the same length, so that, by changing the index of refraction in one of the arms, light can be amplitude modulated at the output. Because matching the precise lengths of waveguide between the two arms corresponding to the appropriate fraction of one wavelength of light is very challenging and is often sensitive to temperature variation, a low speed phase tuning element is commonly required within one arm of the Mach-Zehnder interferometer, providing a static adjustment of the index of refraction to tune the bias point of the modulator.
This tuning can be performed, among other ways, through the use of the aforementioned phase shifter diode by applying to it an appropriate tuning voltage in tandem with the AC modulation voltage. Since the semiconductor's thermal response time is orders of magnitude slower than its electronic response time, the tuning can be performed using essentially a DC voltage where feedback from the modulator's optical output is used to control the amplitude of this DC tuning voltage.
Conventionally, there will be a diode in one arm of the Mach-Zehnder, and this diode will receive a voltage signal that contains both a DC and an AC component. The DC component performs the tuning by setting the phase of the arm in which it resides to be equal to or different by a constant offset to the other arm. The AC component provides perturbation of the index of refraction corresponding to optical modulation at the output of the modulator.
As illustrated in FIG. 1, conventionally a single diode 1 has been used to perform both the tuning and modulation functions. This requires combining the DC tuning components, such as low-speed tuning circuit 2, with the AC modulation components, such as high-speed data source 3, using an integrated bias-tee element.
However, given the inherent tradeoffs discussed previously in selecting the diode operation region, using a reverse-biased diode that has been optimized for high-speed modulation to perform the low-speed tuning operation in addition to the high-speed modulation may result in poor efficiency in terms of power, area, or both.